Metal-foil-assisted fabrication of thin-silicon solar cell

ABSTRACT

One embodiment relates to a method of fabricating a solar cell. A silicon lamina is cleaved from the silicon substrate. The backside of the silicon lamina includes the P-type and N-type doped regions. A metal foil is attached to the backside of the silicon lamina. The metal foil may be used advantageously as a built-in carrier for handling the silicon lamina during processing of a frontside of the silicon lamina. Another embodiment relates to a solar cell that includes a silicon lamina having P-type and N-type doped regions on the backside. A metal foil is adhered to the backside of the lamina, and there are contacts formed between the metal foil and the doped regions. Other embodiments, aspects and features are also disclosed.

TECHNICAL FIELD

Embodiments of the subject matter described herein relate generally tosolar cells. More particularly, embodiments of the subject matter relateto solar cell fabrication processes and structures.

BACKGROUND

Solar cells are well known devices for converting solar radiation toelectrical energy. A solar cell has a front side that faces the sunduring normal operation to collect solar radiation and a backsideopposite the front side. Solar radiation impinging on the solar cellcreates electrical charges that may be harnessed to power an externalelectrical circuit, such as a load.

Solar cell fabrication processes typically include numerous stepsinvolving masking, etching, deposition, diffusion, and other steps.Embodiments of the present invention provide advantageous solar cellprocesses.

BRIEF SUMMARY

One embodiment relates to a method of fabricating a solar cell. Asilicon lamina is cleaved from the silicon substrate. The backside ofthe silicon lamina includes the P-type and N-type doped regions. A metalfoil is attached to the backside of the silicon lamina. The metal foilmay be used advantageously as a built-in carrier for handling thesilicon lamina during processing of a frontside of the silicon lamina.

Another embodiment relates to a solar cell that includes a siliconlamina having P-type and N-type doped regions on the backside. A metalfoil is adhered to the backside of the lamina, and there are contactsformed between the metal foil and the doped regions.

Another embodiment relates to a method of fabricating a solar cell thatinvolves adhering a metal foil to a backside of a silicon substrate. Asilicon lamina may then be separated from the backside of the siliconsubstrate. The metal foil is used as a built-in carrier for handling thesilicon lamina during processing of a frontside of the silicon lamina.

These embodiments and other embodiments, aspects, and features of thepresent invention will be readily apparent to persons of ordinary skillin the art upon reading the entirety of this disclosure, which includesthe accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the subject matter may be derived byreferring to the detailed description and claims when considered inconjunction with the following figures, wherein like reference numbersrefer to similar elements throughout the figures. The figures are notdrawn to scale.

FIGS. 1-6 are cross-sectional views schematically illustratingfabrication of a solar cell in accordance with an embodiment of thepresent invention.

FIG. 7 is a flow diagram of a method of fabricating a solar cell inaccordance with an embodiment of the present invention.

FIG. 8 is a flow diagram of a method of fabricating a solar cell inaccordance with an alternate embodiment of the present invention.

FIG. 9 is a cross-sectional view of a fabricated solar cell asfabricated in accordance with the method of FIG. 8.

FIG. 10 is a planar view of a metal foil over the backside of a siliconlamina in accordance with an embodiment of the present invention.

FIG. 11 is a flow diagram of a method of fabricating a thin-siliconsolar cell in accordance with another embodiment of the presentinvention.

DETAILED DESCRIPTION

In the present disclosure, numerous specific details are provided, suchas examples of apparatus, structures, materials, and methods, to providea thorough understanding of embodiments of the invention. Persons ofordinary skill in the art will recognize, however, that the inventioncan be practiced without one or more of the specific details. In otherinstances, well-known details are not shown or described to avoidobscuring aspects of the invention.

The present disclosure provides techniques for forming thin-siliconsolar cells using a metal foil. Advantageously, the metal foil may beused as a built-in carrier for handling the otherwise fragile siliconlamina during processing of a frontside of the lamina. Subsequently, themetal foil may be re-used to form metal fingers and contacts to theP-type and N-type emitters on the backside of the lamina.

FIGS. 1-6 are cross-sectional views schematically illustratingfabrication of a thin-silicon solar cell in accordance with anembodiment of the present invention. Shown in FIG. 1 is a siliconsubstrate 102 having formed on it a P-type doped (P+) region 104 and anN-type doped (N+) region 106 formed on the backside of the substrate102. The P+ and N+ doped regions may be referred to as P-type and N-typeemitters in the context of the solar cell being fabricated. In thebackside contact solar cell, which is shown in FIG. 1, the emitters andcorresponding contacts are on the backside of the solar cell. The dopedregions may be formed, for example, by diffusing dopants from dopantsources.

A thin dielectric layer 108 may be formed over the P+ and N+ regions onthe backside for electrical insulation, passivation, and/or otherpurposes. The dielectric layer 108 may comprise, for example, siliconoxide and/or silicon nitride. Alternatively, the emitter surface may bepassivated by means other than forming the dielectric layer 108, such asby chemical passivation, for example.

The solar cell structure of FIG. 1 may be placed in an ion implantationtool, which is also referred to as an “ion implanter.” The implanter maybe used to implant ions at a predetermined implant depth 202, asdepicted in FIG. 2. The ions may be hydrogen ions (i.e. protons). Inalternate embodiments, other ions may be implanted or co-implanted withthe hydrogen. For example, helium ions may be implanted instead ofhydrogen ions, or may be co-implanted with hydrogen ions. The dose ofthe implantation induces defects at the implant depth so that the planarlamina of silicon above the implant depth may be separated or exfoliatedfrom the remainder of the silicon substrate below the implant depth. Theenergy of the implantation controls the implant depth and so controlsthe thickness of the thin silicon substrate after the exfoliation. Forexample, the energy of the implantation may be calibrated to cleave athin lamina with a thickness within a range from 10 microns to 100microns. The exfoliation may be accomplished by heating the substrate atan elevated temperature.

As depicted in FIG. 3, a metal foil 306 may be adhered to the dielectriclayer 108 on the backside of the silicon lamina 302. The metal foil 306may be an aluminum foil. To facilitate the foil being used as a carrierfor the lamina, an extended area (handling area) of the metal foil 306may extend beyond a perimeter of the silicon lamina 302. In an exemplaryimplementation, the composition of the metal foil 306 may be Al-1%Si(99% aluminum and 1% silicon), or more generally Al-x%Si, where x% isfrom 0% to 3%. Other compositions for aluminum foil may be used. It isalso possible to use metal foils other than aluminum, such as silverfoil, for example.

In one embodiment, an adhesive layer 304 may be used to adhere the metalfoil 306 to the backside of the silicon lamina 302. The adhesive layer304 may be a thin layer of epoxy, silicone, ethelyne vinyl acetate (EVA)or other encapsulant material which is applied to the backside of thesubstrate. In one implementation, the adhesive layer may be a coatingpre-applied to the metal foil prior to the adhesion.

In an alternate embodiment, the metal foil 306 may be adhered to thebackside of the substrate using an array of contact spots between themetal foil 306 and the backside of the substrate. The contact spots maybe formed by spot melting of the metal foil using a pulsed laser, forexample. In this embodiment, the adhesion layer 304 is not needed. Airgaps beneath the foil between the contact spots may be removed byflattening the foil.

As depicted in FIG. 4, using the foil as a built-in or integratedcarrier to support the lamina, the surface 402 at the front-side of thelamina 302 may then be textured and passivated. The surface texturingserves to increase the capacity of the silicon surface to absorb light,and the surface passivation serves to reduce charge recombination at thesurface. The surface texturing may be accomplished using a wet surfaceetching process, for example. The surface passivation may beaccomplished by chemical passivation or by other means.

Thereafter, a glass encapsulation process may be performed on thefrontside of the silicon lamina 302. FIG. 5 shows the resultant glasslayer 502 which is attached to the frontside using encapsulant material503.

As shown in FIG. 6, further steps may then be performed on the backsideof the silicon lamina 302. These steps include forming metal contacts604 and 606 in contact holes to electrically couple to corresponding P+regions 104 and N+ regions 106, respectively. A first set of metalcontacts 604 may be from the metal foil 304 to the P+ region 104, and asecond set of metal contacts 606 may be formed from the metal foil 304to the N+ region 106. In one embodiment, the metal contacts 604 and 606may be formed using a laser-based contact formation process. In such aprocess, a laser scanner may controllably scan a pulsed laser beamacross the backside of the solar cell being fabricated. The pulsed laserbeam may form the contact openings through the adhesive layer 304 andthe dielectric layer 108, and the contact openings may be filled bymelted metal from the foil 306.

In addition, a finger separation 608 pattern may be formed on the foilarea to electrically separate the first set of metal contacts 604 fromthe second set of metal contacts 606. The finger separation 608 may beconfigured so that the fingers of the foil that lead to the contacts areinterdigitated.

FIG. 7 is a flow diagram of an exemplary method 700 of fabricating athin-silicon solar cell in accordance with an embodiment of the presentinvention. In the exemplary method 700 of FIG. 7, emitter regions may befirst formed on a silicon wafer per block 702. The silicon wafer may beof a thickness of several hundred microns or more and may be referred toas a thick handle wafer. The emitter regions include both P-doped andN-doped regions and may be formed on the backside of the wafer as shownin FIG. 1.

Per block 704, a thin silicon lamina may be cleaved from the siliconwafer. For example, the silicon lamina may be of a thickness between 10microns to 100 microns. In one implementation, the cleaving may beperformed using ion implantation and exfoliation as described above inrelation to FIG. 2. Alternatively, the cleaving may be performed byspalling or etching a sacrificial layer from the frontside of the wafer.

In block 706, metal foil may be adhered to the silicon lamina, asdescribed above in relation to FIG. 3. In particular, the metal foil maybe adhered to the backside surface of the silicon lamina. The metal foilmay be of a thickness between 50 microns and 1 millimeter so as toprovide mechanical support for the thin silicon lamina. To facilitatethe foil being used as a carrier for the lamina, an extended area(handling area) of the metal foil may extend beyond a perimeter of thesilicon lamina. In one implementation, the adhesion may be accomplishedby using a laser to fire contacts between the metal foil and the siliconlamina. In another implementation, the adhesion may be accomplishedusing a thin adhesive layer coated on the metal foil.

Per block 708, the metal foil may be used as an integrated carrier forhandling the silicon lamina so that the frontside surface of the siliconlamina may be processed. The frontside surface processing may includetexturing and passivation, as described above in relation to FIG. 4. Thesurface texturing and passivation may be accomplished, for example, bydipping the lamina into chemical solutions to etch and passivate thefrontside surface. Subsequently, the metal-foil-supported silicon laminamay have its frontside processed with a glass lamination procedure, asdescribed above in relation to FIG. 5. Subsequent to the frontsideprocessing, the extended area (handling area) of the metal foil may betrimmed.

Per block 710, contacts may be formed from the metal foil to the emitterregions. As described above in relation to FIG. 6, the contacts formedmay include a first set of contacts 604 to P-doped emitter regions 104and a second set of contacts 606 to N-doped emitter regions 106. Inaddition, a finger separation 608 pattern may be formed on the foil toelectrically separate the first set and the second set of contacts.

In an alternate embodiment, instead of adhering a continuous metal foillayer to the backside and subsequently creating the finger separationpattern while the foil is attached to the backside, the fingerseparation pattern may be pre-formed in the metal foil before the metalfoil is applied to the backside of the silicon lamina. FIG. 8 is a flowdiagram of an alternate method 800 of fabricating a thin-silicon solarcell which uses such a pre-patterned metal foil in accordance with anembodiment of the present invention.

As shown in FIG. 8, after the thin silicon lamina is cleaved from thewafer per block 704, a pre-patterned metal foil may be sandwiched 806between the backside of the silicon lamina and a secondary substrate.The patterning of the metal foil achieves the finger separation betweenthe P-type and N-type contacts. The secondary substrate may betransparent such that laser light may be transmitted through it. Thesecondary substrate may be, for example, a stiff polymer layer, such asa polyethylene terephthalate (PET) layer or a fluoropolymer layer.Thereafter, per block 807, the contacts may be formed between the metalfoil and the emitter regions. The formation of the contacts may beaccomplished, for example, using a pulsed laser which is transmittedthrough the secondary substrate to create the contact openings and flowmelted metal from the foil into those openings. Per block 708, the frontsurface may then be processed, as described above in relation to FIG. 7.Subsequent to the frontside processing, the extended area (handlingarea) of the metal foil may be trimmed

FIG. 9 is a cross-sectional view of a fabricated thin-silicon solar cellas fabricated in accordance with the method 800 of FIG. 8. As depictedin FIG. 9, the metal foil 306 with the pre-patterned finger separation908 is sandwiched between the secondary substrate 902 and the backsideof the silicon lamina 302. In addition, a P-type contact 904 and anN-type contact 906 are shown. As described above, these contacts may beformed by transmission of a pulsed laser through the transparentsecondary substrate 902.

FIG. 10 is a planar view of a metal foil over the backside of a siliconlamina in accordance with an embodiment of the present invention. Theview of FIG. 10 shows a portion 1004 of the foil over the backside ofthe lamina and an extended area 1006 of the foil which extends beyond aperimeter 1002 of the lamina. Note that the extended area 1006 mayextend over one or more sides of the perimeter and need not necessarilyextend over all sides of the perimeter.

FIG. 11 is a flow diagram of a method 1100 of fabricating a thin-siliconsolar cell in accordance with another embodiment of the presentinvention. In the exemplary method 1100 of FIG. 11, a sacrificial layermay be formed on a silicon substrate per block 1102.

The sacrificial layer may be composed of porous silicon, such as formedin a HF bath with bias. Alternatively, the sacrificial layer may besilicon with, for example, germanium doping and/or a carbon doping,either of which can be formed by epitaxial deposition or a chemicalvapor deposition (CVD) process. The sacrificial layer may be thin, onthe order of approximately 700 micrometers, although it may be slightlyor significantly larger or smaller, as desired for a particularembodiment to perform the functions described herein. For example, incertain embodiments, the sacrificial layer may be as thin as 10micrometers. Smaller thicknesses may also be used in certain instances.

An epitaxial layer of silicon may then be grown over the sacrificiallayer per block 1104. The emitter regions may be formed in the epitaxiallayer per block 1106, and a dielectric layer may be formed over theemitter regions per block 1108.

A metal foil may then be adhered over the emitter regions per block1110. Subsequently, epitaxial lift-off per block 1112 may be performedby selective wet etching or otherwise removing the sacrificial layer.After lift-off, the epitaxial layer becomes the silicon lamina of thesolar cell. A cross-sectional view of the structure at this point in theprocess corresponds to the view shown in FIG. 3. As disclosed herein,the metal foil provides structural support and an integrated carrierfunctionality to the silicon lamina.

Subsequently, the front surface may be processed per block 708. Thecontacts between the metal foil and the emitter regions may then beformed per block 710. In other words, after the epitaxial lift-off perblock 1110, the processing may proceed as described above in relation toFIGS. 4-6.

Techniques for forming thin-silicon solar cells using a metal foil havebeen disclosed. Advantageously, the metal foil may be used as a built-incarrier for handling the otherwise fragile silicon lamina duringprocessing of a frontside of the lamina. Subsequently, the metal foilmay be re-used to form the P-type and N-type emitter contacts and metalfingers on the backside of the lamina.

While specific embodiments of the present invention have been provided,it is to be understood that these embodiments are for illustrationpurposes and not limiting. Many additional embodiments will be apparentto persons of ordinary skill in the art reading this disclosure.

What is claimed is:
 1. A method of fabricating a solar cell, the methodcomprising: cleaving a silicon lamina from a silicon substrate, whereina backside of the silicon lamina includes P-type and N-type dopedregions; and positioning a metal foil between a secondary substrate andthe backside of the silicon lamina; and forming contacts between themetal foil and the doped regions by transmitting a pulsed laser throughthe secondary substrate.
 2. The method of claim 1, wherein an extendedarea of the metal foil extends beyond a perimeter of the silicon lamina,further comprising: using the metal foil as an integrated carrier forhandling the silicon lamina.
 3. The method of claim 1, furthercomprising: forming a first set of contacts between the metal foil andthe P-type doped regions; and forming a second set of contacts betweenthe metal foil and the N-type doped regions.
 4. The method of claim 1,further comprising: forming a finger separation pattern in the metalfoil.
 5. The method of claim 4, wherein the finger separation pattern ispre-formed in the metal foil prior to attachment to the backside.
 6. Themethod of claim 5, further comprising: attaching the metal foil to asecondary substrate prior to attachment to the backside, wherein thesecondary substrate is transparent to a laser light.
 7. The method ofclaim 1, wherein the metal foil is attached to the backside using anadhesive layer.
 8. The method of claim 1, wherein the metal foil isattached to the backside at an array of contact spots between the metalfoil and the backside of the silicon substrate, and wherein the contactspots are formed by spot melting of the metal foil.
 9. The method ofclaim 1, wherein the metal foil comprises aluminum.
 10. The method ofclaim 1, further comprising: texturing and passivating a frontside ofthe silicon lamina while using the metal foil as the carrier forhandling the silicon lamina; and encapsulating the frontside of thesilicon lamina.